This invention relates to a process for removing chlorides from crude petroleum.
Crude oils, including heaving oils and bitumen, are generally found in reservoirs in associating with salt water and gas. As the reservoir becomes depleted, the oil/water interface in the reservoir rises and at some stage, water is coproduced with the oil.
The mixture of water and oil is subjected to a high degree of turbulence during production and these actions form an emulsion in which water droplets are dispersed throughout the crude oil phase. The presence of indigenous surfactants in the crude oil also stabilizes the emulsion by forming a rigid interfacial layer which prevents the water droplets from contacting an coalescing with one another.
Crude oils may, in fact, contain a variety of organic and inorganic contaminants which have detrimental effects on process equipment and operation of a refinery. Organic contaminants may cause unpredictable rates of corrosion in processing equipment and organic contaminants are also a major problem. Normally crude oil contains about 0.01-1% by weight or more of basic sediment, i.e. finely divided sediment. These are water insoluble, inorganic sediments such as sand, silt, clay and gypsum. Although they are relatively inert, they are extremely abrasive. Particle sizes of the basic sediment ranges from 20 to 200 .mu.m. Large particles can be centrifuged from the crude oil and small particles can be separated from the crude oil by electrostatic desalting operations.
In addition, crude oil may contain small particles of metal oxides and sulphide salts termed "filterable solids" They are typically 1 to 20 .mu.m in diameter and insoluble in oil and water. They tend to accumulate at the water/oil interface and act to stabilize the emulsions. These cannot readily be removed from crude oil in a desalting operation without adding an appropriate water wetting agent.
The saline or brine water combined with the crude oil contains various alkali salts forming part of the water/oil emulsion. A typical brine water may contain sodium, calcium, magnesium and potassium in the form of chlorides. Alkali metals are much more concentrated in brine than in sea water and, for example, sodium ions are two to eight times more concentrated in oil field brine water than in sea water. Although the water-in-oil emulsions are stabilized by a large number of contaminants, normal desalting by fresh water removes most of the salts. Sodium hydroxide, often used in crude oil pretreatment, readily reacts with naphthenic acid to form sodium napthenates that contribute to emulsion stabilization.
Ordinarily, commercial desalting operations can remove most of the water soluble contaminants (salts, acids, bases) water insoluble contaminants (basic sediment and filterable salts) and brine water from the crude. Remaining sodium chloride is thermally stable at the temperatures found in the traditional refinery operations, such as crude and vacuum unit furnaces, and has not been a serious problem.
However, with the recent trend of using hydrogenation technologies for upgrading heavy oils, there has arisen a need to reduce the chloride level in the oils to as low as a few ppm. Chloride ion, if accumulated to a certain level, may cause corrosion which is often characterized by the premature failure of reactors and associated vessels. Particularly when a high pressure and temperature hydrogenation process is used, it is essential to assure a very low chloride level in the feed oils.
As noted above, chloride reduction from crude oil may be achieved by removing chloride retaining water droplets. When water droplets are removed, the chloride level comes down as well. Water reduction processes are commonly known as "dehydration" processes. There are several commercial dehydration technologies in use in refineries as follows:
1. Gravity Separation with Demulsifier
Gravity can induce phase separation when a chemical destabilizer (demulsifier) is added to the water-laden crude. The separation is accomplished in large tanks which provide sufficient residence time, often in the order of hours and even days.
2. Gravity Separation with Demulsifier and Viscosity Reduction
The settling velocity of water droplets can be increased by heating the crude oil to reduce the oil viscosity in which water droplets settle by gravity.
3. Centrifugation
The application of centrifugal force can also accelerate the settling velocity of water droplets by increasing effective gravitational field.
4. Gravity Separation with Demulsifier and an Electrostatic Field
The application of high alternative voltage electrostatic field (typically 4 to 5 kilovolts/cm) induces charge separation upon a water surface. As a result, any two adjacent water droplets will collide by attractive force and grow to a larger water droplet, and thus reducing residence time to tens of minutes instead of hours and days. Water droplets may grow from 5 .mu.m to 100 .mu.m, resulting in rapid dehydration.
Although the petroleum industry may employ a variety of techniques (chemical, mechanical or electrical) singly or in combination to effect separation of gross amounts of water from production fluids, the electrostatic approach is almost always selected to remove salt and sediment down to the lowest level required for refining. A typical desalting process uses water-washing followed by induced dipole coalescence and precipitation. This involves the addition of a small amount (typically 5 vol %) of a low chloride water to the crude oil, followed by the intimate mixing of the added water into the oil so as to create a fine dispersion of fresh water droplets among the residue brine droplets and sediment in the crude oil, and finally introducing this dispersion into an intense electric field which accelerates coalescence of the dispersed water droplets and brine droplets, resulting in rapid phase separation. This combination removes 90 to 95% of the incoming salt down to 10 ppm Cl level. Even lower levels can be achieved if two stage desalting (double desalting) is used. An additional 80 to 90% desalting can be achieved resulting in 0.5 to 1.0 ppm Cl levels. However, the double desalting process requires substantial capital expenditures.
When a brine-in-oil emulsion is extremely small, i.e. microemulsion or micelle, it becomes extremely stable and the normal gravitation methods of separation do not work. Even if a centrifuge is used, either processing time or centrifugal force must be substantially increased, or a combination of both of these must be used. This is because the settling rate of a water droplet-through oil is proportional to power two of the droplet diameter. Thus, if the droplet diameter is only one-tenth of a reference droplet, the settling rate of the smaller droplet is only one-hundredth of that of the reference droplet. The settling rate of a droplet through oil is also linearly proportional to the gravitational force. This means that the centrifugation on the smaller droplet must increase by 100 times in order to match the settling rate of the reference droplet.
The application of an electrostatic field normally works well by growing brine droplets by coalescence. When an alternating electric field is applied to the water droplets dispersed in oil, dipole appears on the droplet surface. As the electric field alternates, the droplets begin to oscillate through the oil at different velocities depending on the droplet diameters. This results in the collision of water droplets and coalescence thereof. Water droplets also go through deformation due to the induced dipole formation on the droplet surface. Normally the dipole on the surface also contributes to the collision of water droplets by attraction and growth. However, because the application of the alternating electric field also creates shearing force on the brine droplets, it is conceivable that depending on the effectiveness and concentration of natural surfactants present in the water-oil interface, the droplets may even break up and become smaller (emulsify) rather than growing.
Briceno et al U.S. Pat. No. 4,895,641 describes a method of desalting crude oil in which the formation of a high internal phase ratio oil-in-water emulsion is effective in removing hydrophilic impurities, such as salts, from viscous oils. When a high internal phase ratio oil-in-water emulsion is formed, the hydrophilic impurities become concentrated at the thin aqueous film surrounding the oil droplets. By further diluting the high internal phase ratio by adding water and breaking the oil-in-water emulsion, clean crude oil can apparently be obtained. It will be noted that this process involves the use of only oil-in-water emulsions.
The primary object of the present invention is to develop a new simple and inexpensive process for removing chlorides (desalting process) which can reduce the cost of oil products and also improve the safety risks associated with hydrogenating chloride-containing oil under high temperature and pressure.